Deposited structures

ABSTRACT

A composition, method and apparatus for ink jet deposition of structures are described. Structures which may be deposited have many advantages as a result of their small scale. This is believed to be a result of the sub-micron sized loading used in the composition. Solid Oxide Fuel Cells (SPFCs) are a particular structure which may advantageously be deposited.

The present invention relates to a composition, method and apparatus forink jet deposition of structures, particularly, although notexclusively, of sub-micron sized structures.

It has been recognised that the materials processing and productiontechniques conventionally used in the manufacture of electrical, opticaland mechanical components places a limitation upon their performance. Inpart, the limitation is believed to be attributed to the particulatesize of materials from which such components are formed. Consequently,there has been much theoretical and practical work aimed at overcomingthe performance disadvantages inherent in traditional materialsprocessing and production techniques. In particular, there has been aconcentration on the development of so-called nano-sized materials, thatis materials whose particulate sizes are below one micron (<1 μm).

Whilst some nano-sized materials have been prepared experimentally andare indeed available. commercially in restricted quantities, theavailability of suitable processing and production techniques remains abarrier to the full scale adoption of the technology. As a result, theanticipated benefits in terms of the improved performancecharacteristics of components manufactured using such materials are notbeing realised. By way of example, one such known manufacturing approachis that of photolithography. However, photolithography requires the useof lengthy, labour intensive processes and expensive patterning masks. Amask must be created for each application and/or device. As a resultphotolithography seems not to meet a primary commercial requirement oflow cost.

It is also the case that in parallel with developments in the field ofnano-material manufacture, there have been advances in the processesapplied to the manufacture of components at the micron and greaterscales. U.S. Pat. No. 5,882,722 describes a thick film formed of amixture of metal powders and metallo organic decomposition compounds inan organic liquid vehicle. The document also sets out a process forapplying such a thick film to a substrate. However, the processessuggested in the document for applying such a film to a substrate suchas screen printing, suffer from the disadvantages identified in generalterms above. Another approach taken by those in the field has been thatof ink jet printing in both so-called direct and indirect formats. Inkjet printing has applications as a deposition technique for materialsconsisting of particles greater than one micron in diameter (>1 μm).Although direct ink jet printing is under investigation by someresearchers, the structures which can be produced are very limited interms of the type of materials which can be deposited and the accuracyof the structures which can be produced. Direct printing uses an inkcontaining a solid loading of the material to be printed, much in thesame way that a graphical ink contains the required pigment.Alternatively a derivative of the required material, such as a salt,oxide or complex, can be used in suspension and printed, for laterconversion to the required material. In some cases, it appears thatthere have been attempts even to utilise nano-sized materials in thedirect ink-jet printing process. For example, U.S. Pat. No. 6,361,161suggests that images may be produced using nano-sized particles.Nevertheless, such techniques do not appear to have been commerciallyadopted, primarily it is believed, owing to the difficulty informulating a suitable ink.

Turning to indirect printing there has been much work directed at aparticular deposition technique which has found favour in the productionof structures as opposed to image formation. The process, which hassimilarities to an investment casting, is used to produce wax mouldswithin which a component is subsequently formed in a separate process.

It is the case that there has been much recent interest in thedevelopment of processes for the creation of so-called nano-structures.A typical nano-structure has dimensions of the order of several micronsand is made up of features an order of magnitude smaller. It is expectedthat such structures will exhibit exotic characteristics which areconsidered to be a function of the small size and particularly the largesurface areas of such materials.

It is well known that many processes have been suggested as a means ofcreating such structures. For the most part these processes have beencomplex, time-consuming and seemingly unsuitable for large-scaleproduction at reasonable cost. Indeed, there have been proposals whichset out methods of developing and building devices by means ofdeposition via printing. U.S. Pat. No. 6,294,401 for example teaches amethod of fabricating active components by printing inks containingnano-materials. EP0955685, on the other hand, teaches methods of screenprinting electrodes on either surface of a solid electrolyte. Finally,US20020098401A1 describes fabrication of a structure using multi-layerdeposition.

In the case of EP0955685 and US20020098401A1, there is disclosed amethod of fabricating a particular class of structure known as a solidoxide fuel cell. A solid oxide fuel cell (SOFC) is a particular class offuel cell in which the functional components are all solid state. Assuch, it may be contrasted with the alkali fuel cell known from thespaceflight programme of the United States of America. SOFCs areconsidered to be one of the most likely contenders for practical powergeneration in static applications and may also prove to have potentialin mobile applications.

Typically, as shown in FIG. 8, a SOFC 800 includes a dense electrolyte801 sandwiched between an anode 802 and a cathode 803. Both electrodes802,803 are sufficiently porous to allow a chemical reaction to takeplace between, on the cathode side of the fuel cell, oxygen and on theanode side, a hydrocarbon fuel. The fuel on the anode side is oxidisedby oxygen ions which travel across the electrolyte 801 from the cathode803. Useful electrical energy is thereby generated and extracted from anexternal circuit 804 connecting the electrodes.

In a practical power unit, a number of such fuel cells will be combinedin a stack which may be planar or of some other geometric configuration.Interconnects are required in such a stack to carry the current in muchthe same way that conventional electrochemical cells are connected toform a battery. In view of the high temperatures currently reachedduring the operation of SOFCs, it is ceramic material interconnects areutilised. An example of such a material is lanthanum chromite.

It has been further recognised that a particular limitation on theperformance of a SOFC is the thickness of the electrolyte. In particularresistance or ohmic losses and thus a reduction in fuel cell efficiency,arise in direct proportion to the thickness of the electrolyte layer.

Thus, according to one aspect of the present invention, there isprovided a solid structure fabrication method, the method comprisingfilling each of a plurality of reservoirs with a selected ink, the inkcontaining a solid material loading of nanosized particles, ejectng aselected ink from a print head connected to a corresponding reservoirtowards a medium surface, the print head and medium surface beingmovable relative to each other in a plane defined by first and seconddirections and in a third direction orthogonal to said plane

Advantageously, there is no requirement for a precursor material.Accordingly, complexities are avoided which are inherent in anyconversion process from a precursor material. Furthermore, because theparticulate size is known at outset of the ink formulation process andsignificantly is amenable to analysis, more confidence can be had in thespecifications of structures fabricated in accordance with theinvention. Preferably, a number of print heads will be available eachconnected to a corresponding reservoir containing an ink used in thefabrication of the structure. Where there is a need for voids,depressions or such like in the structure, then a reservoir may befilled with a fugitive material. Typically, the fugitive material isremoved in a subsequent step such as sintering, firing or the like. Asintering step will, of course, be required when a ceramic material isdeposited. Whilst such a sintering step could take place after thedeposition of each ceramic layer, it is preferable to carry outsintering once substantially all the layers, including those layerscontaining ceramic materials, have been deposited.

Preferably, the method permits the selective deposition of material in alayer such that a set of graded layers may be deposited. A structuregraded in this manner can confer benefits in terms of reducing anymismatch between thermal expansion rates of different loadings in theseparate layers. This is particularly advantageous during a sinteringprocess and indeed subsequently in applications of the structure, suchas SOFCs where elevated temperatures are reached during service.

It will be recognised that unlike indirect deposition techniques, thepresent invention facilitates the introduction of interconnects duringthe fabrication process. This capability is advantageous in that it mayremove some difficulties traditionally present in post fabricationprocesses such as sintering and the like.

In accordance with a further aspect of the invention, there is provideda method of fabricating a solid oxide fuel cell, the method comprisingfilling each of a plurality of reservoirs with a selected inkcorresponding to an anode, electrolyte and cathode material, each inkcontaining a solid material loading of nanosized particles, wherein thesolid oxide fuel cell is generated as a plurality of layers, each layerbeing laid down by ejecting at least one selected ink towards a mediumsurface such that an electrolyte layer separates a cathode and anodelayer to form a cell.

It is advantageous if the anode can be built up into a layer havingsufficient structural integrity to support the electrolyte and cathodelayers. The electrolyte layer itself may be deposited as a very thinlayer having a thickness of around 100 or less microns so as to minimiseohmic losses in the completed fuel cell. Furthermore, unlike indirectdeposition techniques there is no restriction on the introduction ofinterconnects during build process. In addition, as a consequence of thereduction in ohmic losses due to the thinner electrolyte layer, the SOFCmay operate at a lower temperature. Consequently, it may be convenientto utilise metallic interconnects. It will be recognised that oneadvantage is that a seal may be more easily formed around a metallicinterconnect. Another advantage of a metallic interconnect is therelative ease, in comparison to a ceramic material, with which aconnection may be formed to circuitry external of the SOFC.

According to another aspect of the invention, there is provided anink-jet deposition apparatus intended for use with above describedmethods to deposit a structure on a medium surface, the apparatuscomprising a plurality of print heads connectable to a selected inkreservoir, the print heads and medium surface being movable relative toeach other in a plane defined by first and second directions and in athird direction orthogonal to said plane.

Preferably, the medium surface is supported on a bed. The bed may befixed, in which case the print heads are translatable in the thirddirection. Alternatively, the bed may be raised and lowered with respectto the print heads providing the relative movement in the thirddirection.

In accordance with a yet further aspect of the invention there isprovided a structure deposited in accordance with one of the abovedescribed methods.

Such structures might include Solid Oxide Fuel Cells (SOFCs), MicroElectro Mechanical Systems (MEMS) and indeed other utilising nanometricmaterial capable of being formulated as an ink composition fordeposition in accordance with the forgoing aspects of the invention.Such structures would provide, advantages in terms of the thindeposition layers achievable. In the particular case of a SOFC thiswould facilitate the creation of solid electrolyte layers having lowohmic losses.

In order to assist in understanding the invention, an embodiment thereofwill now be described, by way of example, and with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic diagram showing an ink jet printer for use inaccordance with an aspect of the present invention;

FIG. 2 is a schematic diagram showing a print head for use with theprinter of FIG. 1;

FIG. 3 is a flow chart illustrative of a method of ink formulation foruse in accordance with an aspect of the invention;

FIG. 4 is a flow chart illustrative of an alternative method of inkformulation for use in accordance with an aspect of the invention;

FIGS. 5 a and 5 b are respectively elevation and plan views of anexample structure deposited in accordance with a method of theinvention;

FIGS. 6 a and 6 b are respectively elevation and plan views of a furtherexample structure deposited in accordance with a method of theinvention;

FIGS. 7 a to 7 d illustrate further examples of a structure inaccordance with a further aspect of theinvention; and

FIG. 8 is a schematic diagram illustrating a prior art Solid Oxide FuelCell.

Referring to FIG. 1, there is shown an ink jet printer 1 under softwarecontrol. The printer 1 is capable of delivering ink to a surface 3 ofmedium 5 which in this case is a polymeric release film. The printer 1is provided with a fixed bed 7 whilst each of a pair of print heads 9a,9 b is capable of movement in the z-plane in addition to movement inthe x and y plane. Each of the print heads 9 is of the piezo-electrictype as exemplified by a commercially available Siemens P2 print head.Clearly, it is envisaged that other ink jet deposition print heads maybe utilised including not only those where the ejection of ink isbrought about as a result of a piezo-electric distortion of an inkcavity but also print heads having thermal or shock wave based ejectionmechanisms. The print heads 9 are fed by separate reservoirs 11 a,11 bto facilitate delivery of different inks without having to repeatedlyflush and refill each reservoir 11 and print head 9 more than necessary.Each print head 9 operates in accordance with a drop on demand processwhereby ink is ejected by the print head 9 solely when it is requiredfor deposition on a medium surface.

Turning to FIG. 2, this illustrates in more detail the print head 9which includes a nozzle 13 of around 18 μm in diameter through whichdroplets of ink are ejected so as to impinge on the surface 3 of themedium 5. Preferably, a print head 9 is selected with a nozzle diameterwhich provides the desired characteristics in both shape and volume ofejected ink. The composition and processing steps required to form anink suitable for printing with the printer 1 are described in detailbelow.

Referring to the flowchart of FIG. 3, an ink containing nano-sizedparticles, i.e. individual particles having a maximum dimension lessthan 1 μm, is formulated by firstly selecting 100 a solid startingmaterial such as, but not limited to, a metal powder, metal salts, metaloxides and ceramic material. Examples of metals include silver,silver/palladium and platinum whilst examples of ceramics include leadzirconate titanate, zirconia and alumina. The individual particlestypically have a size in the range of 2 μm to 10nm.

To the starting material or solid loading as it may also be described,is added 102 a solvent carrier. Typically, the solvent carrier willcontain between 5% and 60% by volume starting material. The solventcarrier must be selected so that it will not destructively interferewith the print head 9 as a result of a chemical process and/ortribological action. Consequently, a solvent such as toluene or acetoneshould be avoided as should certain types of starting material whichhave a tribolgical impact, unless, of course, such wear is deemedacceptable. Similarly, the starting material should be selected suchthat it does not exhibit electrostatic or Van der Waals forces which aresufficient to bring about agglomerations of the starting material whichmight interfere with the operation of the print head 9 through theformation of blockages, for example. The solvent should also be selectedfor its ability to wet the print head 9 and also with a view to definingthe drying time of the ink once in contact with the medium 5. The choiceof an aqueous or non-aqueous solvent will, again, depend on the natureof the starting material. Examples of non-aqueous dispersants includeethyl-lactate and those which are alcohol based including combinationsof ethanol and propan-2-ol, ethylene glycol and other alcohols. In thecase of an aqueous solution it has been found necessary to add a smallamount of an alcohol such as ethanol to provide the wettingcharacteristics necessary to ensure the final ink composition is capableof wetting the print head 9.

In addition to the solvent, it has also been found advantageous to add104 a dispersant or a surfactant to the mixture of the solid materialand solvent. It will be appreciated that a surfactant is particularlysuitable, of course, for use with an aqueous solvent. The molecularstructure of the dispersant or surfactant is such that each molecule hasone end compatible with the material and another end which is compatiblewith the solvent. As a result, the dispersant or surfactant binds thesolvent to the material. The choice between a surfactant or a dispersantwill depend on the nature of the interface which is to be formed betweenthe constituents of the composition. A dispersant is, of course, capableof forming interfaces between solid and liquid phases only, whereas asurfactant can not only form interfaces between solid and liquid phasesbut also between solid and solid, solid and liquid, solid and gas,liquid and liquid and liquid and gas phases.

One example of a formulation which has achieved favourable results isone containing 5% by volume silver oxide, EFHK 440 as a dispersant at 2%by weight of the silver oxide mass and the remainder being anethanol/propanol solvent carrier.

The resulting mixture is then homogenised 106 using a process such asmilling. The process may be carried out for a number of hours.Typically, three hours is sufficient.

In an alternative embodiment of the present invention (see FIG. 4), thedispersant or surfactant is added 200 to the starting material and bothare mixed 202, typically the dispersant or surfactant is mixed by handwith the starting material. To the homogenised mixture is then added 204sufficient solvent such that the starting material makes up between 5 to60% by volume of the resulting mixture. The resulting mixture may thenhomogenised 206, preferably through a further milling process for amatter of hours perhaps three hours.

It has also further been determined experimentally that in order toavoid cavitation or blockages within the nozzle 13, it is important tocontrol the viscosity of the ink whilst it passes through the print head9. Preferably, the viscosity of the ink will be in the range of 10-60cPsat ambient temperature namely at a temperature in the range of around16° C. to 35° C. More preferably, the viscosity will be selected to bein the range of 20-50 cPs.

Typically, a manufacturer of a print head 9 will provide a range ofviscosities which it is considered by the manufacturer are appropriatefor an ink to be successfully deposited from the print head 9.Surprisingly, it has been found that inks in accordance with presentinvention may still be printed successfully despite having a viscositylaying outside the range specified by the print head manufacture. It isbelieved that this is because the inks types considered by themanufacturer when determining the recommended viscosity range differsignificantly in their desired characteristics from those of the presentinvention. To take one example, whilst drying time is a significantattribute in relation to known inks suited for conventional printingoperations, this is not the case with inks of the present inventionwhere drying times may be much more extensive. In addition, the natureof the medium 5 onto which the ink may be ejected from the print head 9is also a factor in the selection of a viscosity or viscosity range forthe ink. By controlling the viscosity of the ink at the point ofdelivery to the medium 5 it is possible to optimise the shape and sizeof a drop of ink to meet the media requirements and to facilitate thebuild-up of a structure.

It has also been found experimentally that when multi-dimensionalstructures are built up using an ink, a lack of physical integrity canarise in the built up structure unless steps are taken to control theintegrity during the build of the structure.

With reference again to FIGS. 3 and 4, in order to address both of theabove issues, it has been found useful to add 108, 208 a furthercomponent to the homogenised mixture, namely a binder. The type andquantity of binder added to the mixture of solvent, starting materialand dispersant or surfactant is again determined by the requiredcomplexity of the built up structure and the factors determining thedesired viscosity set out above. The binder itself has to be soluble inthe selected solvent and removable from the built up structure by a postprinting process such as leaching or firing, for example. Some suitablebinders have been found to be polyvinylalcohol (PVA) andpolyvinylbutryol (PVB) for non-aqueous alcohol based solvents. Latex hasbeen found to be a suitable binder for aqueous solvents.

The final step 110,210 in the preparation of the ink is to subject it toagitation in order to break down any tendency for the material toagglomerate. It has been found that ultrasonic techniques such as theuse of an ultrasonic probe also known as a horn or alternatively anultrasonic bath are effective in breaking down any agglomerates. It isbelieved that the tendency for the starting material to agglomerate isdue to Van der Waals forces which are interactions between closed-shellmolecules and have contributions from interactions between the partialelectric charges of polar molecules. Typically, the period required forultrasonic agitation to achieve the result of breaking down large scaleagglomerations is up to five minutes or so, preferably around twominutes.

It has been found useful to carry out such agitation 110,210 immediatelyprior to viscosity testing of the ink and also before utilising the inkin the deposition process set out in more detail below.

Once the ink has been agitated and any large agglomerations broken down,it has been found beneficial to use 112,212 the ink as soon as possibleso as to minimise the opportunity for the material to agglomerate and asediment to form. Nevertheless, it has been determined that ink preparedin the above manner can be used at a later date provided agitation110,210 is carried out to remove any sediment which has formed. It isexpected that an ink formulated in the above-described manner willbecome fully sedimented in no less than about six months. Accordingly,an ultrasonic probe 15 maybe incorporated in the reservoir 11 within theprinter 1 itself, the agitated ink being subsequently delivered to theprint head 9.

In use, the reservoir 11 of the printer 1 is filled with ink prepared inaccordance with the above procedure. The printer 1 itself, as has beenmentioned, is capable of delivering ink to a medium 5 placed on the bed7 at a particular position defined by the x and y co-ordinates.Furthermore, because the bed 7 itself may be moved in the z direction itis possible to deposit ink onto the medium 5 at a number of x and yco-ordinates and at a fixed z position before displacing the bed 7 inthe z direction and again depositing material at selected x and yco-ordinates. In this manner, it is possible to build up a structure 500on the medium 5 having a three-dimensional structure (FIGS. 5 a and 5b). Clearly, a two dimensional structure 600 (FIGS. 6 a and 6 b) can becreated by depositing the ink over the medium 5 with the bed 7 held in afixed position relative to the print head 9.

It will be recognised that control of each print head 9 and bed 7 may beplaced under software control. Consequently, Computer Aided Design (CAD)software may be utilised to generate the design of a structure which canthen be utilised in Computer Assisted Manufacture (CAM) of the structureby the printer. For example, the design of the structure may be createdvia a pixellated bit map. The software interprets the bitmap such thatone pixel of the bitmap represents one ink drop. A three dimensionalstructure may be built up by referring to a superimposed set of suchbitmaps. This allows unique structures to be designed and produced on adrop by drop basis enabling complex geometries and hybrid structures tobe realised.

FIGS. 7 a to 7 d illustrate in cross-section how a number of differentstructures 700 may be built up from a series of layers deposited ontothe polymeric release film 5. The particular solid loads used in theinks deposited in the structures 700 described below will depend, ofcourse, on the function of the structure 700. For example, a Solid OxideFuel Cell will include an anode, an electrolyte and a cathode as well asany interconnects required to facilitate formation of a stack.

In the Figures that follow, the particular geometries are intended to beexamples of the sort of complex structures that can be achieved such asmight be applied to a Solid Oxide Fuel Cell or a micro electromechanical system (MEMS), to take just two such types of device.

In FIG. 7 a, a first layer 701 is deposited directly onto the releasefilm 5. The first layer 701 is of constant thickness and is deliveredfrom a first reservoir of ink 11 containing a predetermined nanometricsolid loading using a print head 9 connected to the reservoir. Thesecond layer 702 deposited on the first layer 701 is built-up byinitially delivering material from a second reservoir 11 using acorresponding printhead 9. However once a certain thickness of thislayer 702 has been achieved, another print head connected to a furtherreservoir 11 containing an ink having a different nanometric solidloading is used to deposit ink in the two regions 703 a,703 b.Ultimately, deposition of material from the second reservoir stops andink from the further reservoir is delivered in an uninterrupted layer704 over the entire cross-section of the device.

Similarly in FIG. 7 b, the inclusion 702 is formed by using both firstand second print heads 9 a,9 b connected to respective reservoirs 11 todeposit the selected inks over the relevant portions of thecross-section of the structure. The inclusion itself may be formed offugitive material such that a void may remain within the cross-sectionof the structure following a post deposition sintering or similaroperation.

In FIG. 7 c, there is shown a graded structure 700 in which an ink isdeposited 703 in a graded amount over the cross section of the structurebuilt up from an initial set of two layers 701,702 each of constantthickness.

In FIG. 7 d, it is shown how with control of the print heads 9 and thereappropriately provisioned reservoirs 11 containing suitably loaded inks701,702 and fugitive material can produce a tubular cross section havinga central portion of fugitive material 703 which can be removed in apost deposition step to form a void.

It will be appreciated that above examples are not intended to belimiting in respect of the type of structure 700 which can be achieved.

Such flexibility in generation of structures is particularly applicableto the creation of Solid Oxide Fuel Cells, where metallic or other formsof interconnect between cells may be deposited together with the otherelements of the stack. As a result, a complete stack can be built-up andsubsequently sintered in a single operation rather than the series oflaying up and sintering operations required in the prior art.

1. A solid structure fabrication method, the method comprising fillingeach of a plurality of reservoirs with a selected ink, each said inkcontaining a solid material loading of nanosized particles, ejecting aselected ink from a print head connected to a corresponding reservoirtowards a medium surface, the print head and medium surface beingmovable relative to each other in a plane defined by first and seconddirections and in a third direction orthogonal to said plane.
 2. Amethod as claimed in claim 1, wherein the solid structure is generatedas a plurality of layers, each layer being laid down by ejecting atleast one selected ink towards the medium surface.
 3. A method asclaimed in claim 2, wherein a contiguous feature of said solid structureis generated by selectively ejecting a selected ink towards the mediumsurface so as to form a set of at least partially superimposed portionsof said layers.
 4. A method as claimed in claim 1, comprising filling areservoir with a fugitive material and ejecting the fugitive materialfrom a print head connected to the reservoir towards the medium surface.5. A method as claimed in any claim 1, wherein the structure isseparated from said medium surface.
 6. A method as claimed in claim 1,comprising selecting the solid material loadings to form a structurehaving an anode, a cathode and an electrolyte.
 7. A solid oxide fuelcell fabrication method, the method comprising filling each of aplurality of reservoirs with a selected ink corresponding to an anode,electrolyte and cathode material, each ink containing a solid materialloading of nanosized particles, wherein the solid oxide fuel cell isgenerated as a plurality of layers, each layer being laid down byejecting at least one selected ink towards a medium surface such that anelectrolyte layer separates a cathode and anode layer to form a cell. 8.A method as claimed in claim 7, wherein the layers are removable fromthe medium surface.
 9. A method as claimed in claim 7, wherein at leastone reservoir is filled with a fugitive material and selectively ejectedtowards the medium surface.
 10. A method as claimed in claim 9, whereina post deposition sintering operation is carried out.
 11. A method asclaimed in claim 7, wherein at least one reservoir is filled with aselected ink corresponding to an interconnect material, the inkcontaining a solid material loading of nanosized particles, wherein acontiguous interconnect feature is generated by selectively ejectingsaid selected ink towards the medium surface so as to form a set of atleast partially superimposed portions of said layers.
 12. A method asclaimed in claim 11, where a stack of solid oxide fuel cells is formedby depositing a plurality of sets of anode and cathode layers eachseparated by an electrolyte layer such that said cells areinterconnected by respective interconnect features.
 13. An ink-jetdeposition apparatus for use in accordance with the method of claim 7 todeposit a structure on a medium surface, the apparatus comprising aplurality of print heads connectable to a selected ink reservoir, theprint heads and medium surface being movable relative to each other in aplane defined by first and second directions and in a third directionorthogonal to said plane.
 14. A structure deposited in accordance withthe method of claim 7, wherein the structure is removable from themedium surface.
 15. A structure as claimed in claim 14, wherein themedium surface is a polymeric release film.